1. Field of Invention
The present invention relates to a surface light source device and liquid crystal display, in particular, to a surface light source device utilizing a light guide plate having a back face provided with a great number of micro-reflectors and a liquid crystal display utilizing the surface light source device for illuminating a liquid crystal display panel.
The present invention is applied, for example, to liquid crystal displays in devices such as personal computers, car navigation systems or portable phones and surface light source devices used therein.
2. Related Art
A well-known surface light source device uses a light guide plate that has an end portion and an emission face, wherein light is supplied and introduced into the light guide plate through the end portion and is outputted through the emission-face, being applied to broad uses such as illumination of liquid crystal display panels.
Although rod-like fluorescent lamps (cold cathode tubes) have been broadly employed as primary light sources, those using point-like light emitter like LED (Light Emitting Diode) have been adopted growingly in recent years.
Surface light source devices of such a type introduce light into a light guide plate to redirect the light toward an emission face from which the light is outputted.
As known well, light-direction-conversion within a light guide plate and emission from an emission face are promoted by employing a light guide plate made of light scattering-guiding material, or by applying emission promoting processing such as making a back face or emission face light-diffusible.
However, as known well, such means causes the emitted light to be preferentially directed to much forward inclined directions (for example, about 60 degrees with respect to a frontal direction). Such largely inclined output directions are much quite different from usually desired output directions, that is, generally frontal directions or around them. According to a prior proposition to realize a direction-conversion capable of providing a preferential output direction which is desired, a great number of micro-reflectors are formed on a back face of a light guide plate.
According to the art using micro-reflectors, they are formed on the back face of a light guide plate like a great number of micro-projections, generating an inner propagation light proceeding toward an emission face by means of inner-face reflection of the projections. This inner propagation light is emitted from the emission face, becoming an output light. Here described is an example of arrangement comprising a light source device, which employs a light guide plate provided with micro-reflectors, for backlighting a liquid crystal display panel by referring to FIGS. 1 to 4.
In the first place, FIG. 1a is a back side plan view of an outlined arrangement of a surface light source device employing a light guide plate provided with micro-reflectors for backlighting of a liquid crystal display panel, and FIG. 1b is a side view from the left side in FIG. 1a. FIG. 2 illustrates an array example of micro-reflector 20 in the arrangement. In these illustrations, a light guide plate denoted by reference numeral 10 is made of a transparent material such as acrylic resin, polycarbonate (PC) or cycloolefin-type resin, a side end face of which provides an incidence face 12.
A rod-like primary light source (cold cathode tube) L1 is disposed along the incidence face 12 which is supplied with light from the primary light source. The light guide plate 10 has major faces 13 and 14 one of which provides an emission face 13. The other face (back face) 14 is provided with a great number of micro-reflectors 20 shaped like micro-projections.
A well-known liquid crystal display panel PL is disposed on the outside of the emission face 13 to provide a liquid crystal display of backlighting type. It is noted that the micro-reflectors 20 are not shown in FIG. 1a. Size values are merely examples, being indicated in mm.
The primary light source L1 emits light, which is introduced into the light guide plate 10 through the incidence face 12. An inner propagation light travels within the light guide plate 10 and undergoes direction-conversion on entering into micro-reflectors 20 through inner-reflections by inner faces of projections, with the result that light proceeding toward the emission face 13 is produced. Such inner reflection occurs twice generally as described later.
An example of arrangement of micro-reflectors 20 on the back face 14 of the light guide plate 10 is shown in FIG. 2. It is noted that the primary light L1 disposed along the incidence face 12 is a rod-like cold cathode tube having an emitting portion length of which is somewhat smaller than that of the incidence face 12.
Both ends are electrode portions EL1 and EL2 which are incapable of emitting light. Such a design is adopted often in order to avoid the electrode portions EL1 and EL2 of both ends from protruding.
Micro-reflectors 20 are distributed on the back face 14 so that covering rate tends to increase according to an increasing distance from the incidence face 12. Further to this, micro-reflectors 20 are arranged in corner area C and D located close to the electrode portions EL1 and EL2, respectively, at a specifically large covering rate.
Such a covering rate distribution prevents brightness from varying depending on distance from the incidence face 12 and from being short in the corner areas. It is noted that “covering rate” of micro-reflectors is defined as area occupied by micro-reflectors per unit area of a back face of a light guide plate.
Each micro-reflector 20 is shaped like a quadrangle-pyramid, projecting from a general plane representing the back face 14 (i.e. a plane formed by provisionally removing the micro-reflectors 20). Each micro-reflector 20 has a posture determined as to cause light approaching there to be inner-inputted efficiently and to be converted into an inner output light proceeding generally at right angles with respect to the emission face 13. Such processes are described with referring to FIGS. 3, 4a, 4b, and 4c. 
FIG. 3 shows one of the micro-reflectors 20 with an illustration of direction conversion of an inner propagation light effected by the micro-reflector. In the illustration, the inner propagation light is represented by representative light beams P1 and P2. Beam P1 represents an inner propagation light which is inner-reflected by the slope 21 and then by the slope 22 in order while beam P2 represents an inner propagation light which is inner-reflected by the slope 22 and then by the slope 21 in order. Beams Q1 and Q2 represent inner output light beams produced from beams P1 and P2, respectively.
It is noted that a pair of beams P1 and P2 run in parallel with a main approaching direction of light which is inner-inputted in a corresponding micro-reflector 20. In FIG. 3, coordinate 0-XYZ is a right-hand coordinate used to denote directions, Z-axis of which extends vertically to the emission face 13 so that +Z-direction corresponds to a “frontal direction”.
X-axis is perpendicular to both Z-axis and the incidence face 12, having an orientation (plus-minus sign) such that +X-direction extends as to get farther from the incidence face 12. Y-axis runs at right angles with respect to both Z-axis and X-axis as to provide a right-hand rectangular Cartesian coordinate 0-XYZ (having original 0 optionally positioned), extending in parallel with the incidence face 12.
For the sake of description in the instant specification, a rectangular Cartesian coordinate 0-xyz, which is independent of coordinate 0-XYZ, is defined for each micro-reflector. Defined are x-axis, y-axis and z-axis as follows.
In the first place, z-axis extends in the same direction (including orientation) as that of Z-axis, having +z-direction which corresponds to the “frontal direction”
A projection of a main approaching direction (including orientation) of light to be inner-inputted into a corresponding micro-reflector onto the emission face gives a direction of x-axis which extends perpendicularly to z-axis. And y-axis runs at right angles with respect to both z-axis and x-axis as to provide a right-hand rectangular Cartesian coordinate 0-xyz (having original 0 optionally positioned).
It should be carefully noted that x-axis may extend in a different direction as compared with X-axis and y-axis may extend in a different direction as compared with Y-axis in general, although 0-xyz accords with 0-XYZ in the case of the micro-reflector illustrated in FIG. 3.
For example, micro-reflectors arranged in the corner portions C and D shown in FIG. 2 give y-axes non-parallel with Y-axis and x-axes non-parallel with X-axis because projections of main approaching directions of inner input light to the micro-reflectors onto XY-plane are inclined with respect to X-axis.
As illustrated in FIG. 3, a micro-reflector 20 has a pair of slopes 21 and 22 located on a side farther from the incidence face 12, the slopes 21, 22 providing a first and second inner-reflection faces. Both slopes (inner-reflection faces; in the same way, hereafter) 21 and 22 form a valley in the light guide plate, meeting each other to form a valley bottom portion 25. Viewing from standpoint outside of the light guide plate, such a bottom portion 25 can be called “ridge portion”.
There are another pair of slopes 23 and 24 located on a side nearer to the incidence face 12, the slopes meeting each other to form a ridge 26. It is noted that a foot line of a micro-reflector 20 (intersection between a micro-reflector and a general plane representing the back face 14) is shown by dotted lines in FIG. 3.
As described above, in this embodiment, each micro-reflector 20 gives the inside of the light guide plate a valley provided by slopes 21, 22 and another valley provided by slopes 24, 25.
Light beams P1 and P2, which represent an inner propagation light approaching a micro-reflector 20 via the incidence face 12, reach one of the slopes 21, 22 of the micro-reflector 20 from the incidence face 12 directly, or after being inner-reflected by the emission face 13 and/or back face 14. It is noted that some light may be directed to the slope 21 or 22 after being inner-reflected by the slope 23 or 24.
A large part of light reaching the slope 21 or 22 is inner-reflected by the slope 21 and then by the slope 22, or by the slope 22 and then by the slope 21, with the result that an inner propagation light proceeding toward the emission face 13 is produced. This light is emitted from the emission face 13 to provide output light Q1, Q2 of the light guide plate 10.
Thus a pair of 21 and 22 of each micro-reflector 20 function as a conversion output portion which inner-outputs light by converting a proceeding direction of an inner-inputted light. It is noted that references Q1 and Q2 are also used to denote emitted beams.
Some consideration is given to postures of micro-reflectors 20 as follows. FIGS. 4a, 4b and 4c illustrate from three directions how light representing beams P1 and P2 inner-inputted to a micro-reflector formed in a standard posture are converted into inner output light Q1 and Q2 proceeding toward a frontal direction.
FIG. 4a gives an illustration viewed from +z-axis direction (the same as +Z-direction due to definition), FIG. 4b gives an illustration viewed from +y-axis direction (the same as +Y-direction in this case), and FIG. 4c gives an illustration viewed from +x-axis direction (the same as +X-direction in this case).
Referring to these illustrations, behaviour of the above-mentioned representing beams P1 and P2 is described again with the use of the coordinate o-xyz.
As shown in FIG. 4a, representing beams P1 and P2 have an approaching direction to a micro-reflector 20 and the approaching direction provides a projection onto xy-plane in a direction consistent with +x-direction. Representing beams P1 and P2 inputted to the micro-reflector 20 are inner-reflected by the slopes 21 and 22 inclined with respect to every one of xy-plane, yz-plane and zx-plane, being converted into beams Q1 and Q2 directed toward +z-direction. This will be understood with ease specifically by referring to FIGS. 4b and 4c. 
These beams Q1 and Q2 represent inner output light, being parallel to each other. Beams Q1 and Q2 are emitted from emission face 13 toward +z-direction.
In the instant specification, if such direction conversion is effected by each micro-reflector having a posture (as shown in FIGS. 4a, 4b and 4c), the posture is called “standard configuration”. Standard configuration requires the following conditions 1, 2 and 3 to be satisfied at the same time.
Condition 1; A projection of an extending direction of, a valley bottom portion 25 onto xy-plane accords with x-axis direction (See FIG. 4a, specifically).
Condition 2; A bisectional plane, which bisects an angle made by a first and second inner-reflection faces 21 and 22 so that the valley bottom portion 25 extends on the bisectional plane (called merely “bisectional plane”, hereafter), is perpendicular to xy-plane (See FIG. 4a specifically). In other words, the first and second inner-reflection faces 21 and 22 are inversely and symmetrically inclined with respect to a plane that passes the valley bottom portion 25 and extends perpendicularly to the emission face 13.
Condition 3; An inner input light inner-inputted to the micro-reflector from a main approaching direction (+x-direction) is converted into an inner output light proceeding toward +z-axis direction.
If a light guide plate has a back face provided with a great number of micro-reflectors 20 arranged in such standard configuration and the light guide plate is used in a surface light source device, primary light supplied sideways is converted directly into inner output light directed to a generally frontal direction which is outputted at a high efficiency, bringing a merit with a simple structure.
However, in prior arts employing micro-reflectors in the standard posture tends to cause the output light to have an excessive directivity, being suffered from a problem that a small deviation of viewing direction from a main emission direction (i.e. direction of Q1 and Q2) brings a sharp reduction in brightness (Narrow viewing angle).
In particular, a posture on z-axis is made fitting in with the above-mentioned Condition 1, there rises a drawback that viewing angle in zx-pane differs much from that in yz-pane and the latter (viewing angle in yz-pane) is very small.
FIGS. 5 and 6 are graphs to illustrate results of simulation calculation of angular characteristics of emission intensity in a case where a micro-reflector is used in the standard posture.
Abscissa in FIG. 5 and inclined-abscissa to the upper right indicate angles (inclination angles) in ZX-plane wherein plotting s with sign − correspond to a nearer side to the incidence face and plotting with sign + corresponds to a farther side from the incidence face.
Ordinate in FIG. 5 and inclined-ordinate to the upper left indicate angles (inclination angles) in YZ-plane, wherein plotting with sign + corresponds to right-handed inclinations as viewed from the incidence face and plotting with sign — corresponds to left-handed inclinations as viewed from the incidence face.
In FIGS. 5 and 6, light intensity after a well-known Cosine correction (correction of values in accordance with cosine of inclinations of a light measuring direction) is illustrated in five discrete-intensity-levels. FIG. 6 is a graph for three-dimensional indication prepared based on the graph of FIG. 5, wherein light intensity is indicated in discrete-density shades and three-dimensional iso-brightness curves, and height from a plane of axes rising to the upper right and to the upper left expresses brightness (light intensity) after Cosine correction.
A set of parameters r, s, t are used as required for indicating a posture of a micro-reflector with respect to the standard configuration. It is noted that direction-angles (degrees) around z-axis, x-axis and y-axis are expressed by r, s and t with respect to those of the standard posture, respectively. Of course, the standard configuration corresponds to r=s=t=0.
It is understood from the graphs that the single emission direction peak is shown as a direction of angles of about zero both in ZX-plane and YZ-plane, namely, being directed to a generally frontal direction. Further to this, FIG. 5 shows a grey-scale pattern shaped like a slender ellipse.
The ellipse has a longitudinal axis that corresponds approximately to +x-direction (inclined about 45 degrees with respect to +X-direction due to simulation condition in this case). Since +x-direction is nothing other than an orientation direction of micro-reflector 20 as viewed from the above the emission face 13 (i.e. an extending direction the of ridge 25), in short, the graphs of FIGS. 5 and 6 tell that the angular extent around the brightness peak direction is far from being isotropic.
Thus it is understood that emission face intensity has a specifically large gradient regarding ±y-directions. This means that only a small angular deviation of viewing from the peak direction (i.e. brightest viewing direction) regarding ±y-directions brings a sharp reduction in brightness, which gives usually undesirable characteristics.
This problem is relaxed to some degree if a light diffusing sheet or light diffusing plate is disposed along the emission face 13 of the light guide plate 10. An example demonstrating this is shown in graphs of FIGS. 7 and 8. FIGS. 7 and 8 show re-plotting under a condition such that light diffusion effected by a diffusing sheet is additionally considered in the simulation corresponding to FIGS. 5 and 6.
Abscissa and ordinate in FIG. 7 indicate the same as those in FIG. 5, and axes inclined to the upper right and left indicate the same as those in FIG. 6. Manners of light intensity indication in FIGS. 7 and 8 are also the same as those in FIGS. 7 and 8, respectively.
It is understood by comparing graphs of FIGS. 6 and 8 with graphs of FIGS. 5 and 7 respectively that there is single peak direction of emission directed to an approximately frontal direction. Further to this, FIG. 7 shows a less slender ellipse shading pattern as compared with that shown in FIG. 5 and that FIG. 8 shows a somewhat gentle rising as compared with a sharp rising shown in FIG. 6, which demonstrate some effectiveness.
However, it involves difficulties to intend to obtain sufficient angular extent of brightness around the peak direction by means of strong light diffusion ability because the stronger light diffusion ability the light diffusion member employed, the more light diffusion toward useless directions occurs.
Although it seems this problem could be avoided by adjusting posture of micro-reflectors so that the above Condition 1 is broken by a small angle, instead of the above way relying upon a light diffusion member, such posture adjusting fails to give a sufficient extent around the peak direction and, if Condition 1 is broken by a large angle, there arises a reduction in emission efficiency. If micro-reflector posture is adjusted as to break the above Condition 2, situation is generally not changed.